Aircraft components such as fuselages, wings, stabilizers, landing gear doors, and flight control surfaces are traditionally constructed of aluminum alloy or other lightweight metals. In order to further reduce weight, increase strength, improve corrosion resistance, and provide other attendant benefits, such components may be alternatively constructed from advanced composite materials. Examples of such advanced composite materials include carbon laminates and carbon sandwich composites, as well as woven or non-woven materials such as KEVLAR, boron, graphite, and fiberglass.
Damage modes for composite structures include delamination/de-bonding, fiber breakage, and matrix cracking. In-situ inspection may be necessary due to the potential for damage to progress due to handling and testing of the composite structure. Fatigue testing of such composite structures is critical to the validation of structural designs and programming of damage prediction models, which require an accurate understanding of the formation and growth of damage so that failure of the composite structure may be accurately predicted.
Conventional inspection methodologies include acoustic emission testing, passive thermography, digital image correlation, and fiber optic sensing. Acoustic emission testing involves the use of sensitive acoustic emission sensors to locate acoustic events. The acoustic events, which are caused by micro-level and macro-level changes in the composite structure, may be associated with certain types of damage. Acoustic emission testing is generally able to detect damage onset, but cannot optimally detect the shape, size, and depth of such damage. However, all sources of acoustic emission do not develop into critical damage.
Passive thermography is a non-contact inspection method that uses infrared cameras to detect localized areas of heating. Such heating can be caused by breaking, rubbing, or clapping together of materials in the damaged areas, and can provide additional information regarding damage location and size. Digital image correlation measures displacement at the surface due to damage under loading, but requires significant amounts of subsurface damage to accumulate before being detectable at the surface. Fiber optic measurement can detect changes in strain in a test sample, but likewise is relatively ineffective in measuring the shape or depth of damage.
Systems and methodologies exist for detecting and quantifying failure events using a combination of sensor technologies of the types described above. For instance, U.S. Pat. No. 7,516,663 to Ringermacher et al. discloses a process for locating a failure event via acoustic emission sensors. Time-based thermography data is then used to study the area of the detected emission event and track the evolution of heat at the location so as to determine a depth of any damage. However, while such an approach takes advantage of the different capabilities of acoustic and infrared sensors, it remains less than optimal for use in wide area in-situ fatigue monitoring in loaded composite structures, as well as for accurately predicting composite failure.